The term capillary structures as used in this invention generally refers to tubes with small bores made of amorphous materials such as quartz or fused silica. U.S. Pat. No. 293,415 teaches the use of silica capillary columns for gas chromatography and teaches manufacturing in much the same way as the production of hollow optical fibers. It is also known from U.S. Pat. No. 5,552,042 to wind silica capillaries on open tubular assemblies such as a fused mandrel to provide, after annealing, a relatively stress-free rigid winding.
Capillary structures are also used in electrophoresis. Electrophoresis methods generally place oppositely charged electrodes at opposite ends of an ionic base to provide an electric field. The applied electric field produces a migration of electrons through the ionic solution that separates a sample as it is dragged along with the migrating charge in the direction of the cathode. General methods for practicing electrophoresis are well known and are briefly described in the text entitled "Practical Capillary Electrophoresis". Capillary electrophoresis (CE) retains the ionic solution, or buffer as it is often called, in the bore of a capillary and places the oppositely charged electrodes at opposite ends of the capillary. As the mobile positive charges migrate through the buffer in the direction of the cathode, sample is again dragged along with the migrating charge through the ionic solution or buffer. Basic capillary electrophoresis is also referred to as capillary zone electrophoresis (CZE). A number of the electrochemical mechanisms may be employed in conjunction with CZE and the use of the term capillary electrophoresis (CE) in this invention is meant to refer generally to any arrangement where an electrical field drives solutes through a capillary or arrangement that uses an electroosmotic flow to separate or isolate components. Types of methods falling in this definition include: capillary isoelectric focusing (CIEF) that uses carrier ampholytes to create a pH gradient through the capillary; Capillary Gel Electrophoresis (CGE) where the capillary passage retains an ionic gel; Capillary Isotachophoresis (CITP) that normalizes the velocity of the sample components through the capillary; Electrokinetic Capillary Chromatography (EKC) that employs a semi-stationary phase in a background electrolyte; and Capillary Electrochromatography (CEC) that employs electroosmotic pumping. Additional information and arrangements for capillary electrophoresis can be generally found in patents such as U.S. Pat. No. 5,045,172 and RE34,757.
Whether used in chromatographic separations or capillary electrophoresis, capillary structures have generally consisted of a single uniform capillary. In some applications, individual capillaries may be grouped together to provide simultaneous sampling. The use of multiple capillaries to analyze single samples presents numerous problems in the handling of multiple thin capillary members along with making connections to capillaries that should simultaneously receive and deliver samples from the small openings of the capillaries.
The only structure that resembles multiple capillaries has been used in image intensifiers. Image intensifiers, also known as a night vision scopes, use a structure comprising bundles of fibers. The method for manufacturing the fiber bundle for a night vision scope draws a circular glass tube through a hex die. The resultant hex shaped tubes with circular holes are then grouped together and drawn together as an assembly to form a fused bundle of reduced cross-section. The external hex shape of individual capillaries facilitates the packing together of a grouping of capillaries., i.e. similar to a bee's honeycomb. It is also known in the art to make a multi-passage capillary assembly with this arrangement by using hollow tubes instead of fibers. The circular holes in such a bundle are ordered with no void space in the walls and the assembly can become very large. A variation of this method casts each starting hexagonal member instead of die drawing. The biggest disadvantage to this approach is the resultant final shape of the assembly which is a hex. In many applications, such as a gas chromatograph, the ends of the multi-capillary need to be attached to other parts; the hex shape causes difficulties in getting compression type fittings to interface.
The problem of making connections to capillary structures is not a trivial one. The fine diameters of tubing and the low tensile strength of capillary column materials, such as fused silica, make the arrangement of capillary columns and of capillary connectors for the capillary tubes especially difficult. Although many methods and procedures for making such connections are possible, the connections generally require bonding to a conduit that has a circular cross-section. Suitable connection arrangements have been described in patents and applications such U.S. Pat. No. 5,692,078 as well as copending applications Ser. No. 60/065,712 and 60/065,711; both provisionally filed Nov. 14, 1997.
In the present state of the art for chromatography, capillary columns can have a larger sample capacity by increasing its bore and applying a thicker stationary phase, or it can have higher plate efficiency by reducing its bore diameter and using a thinner stationary phase. A way of getting both benefits simultaneously is to use a multi-capillary. A multi-capillary is a grouping of many capillaries all bundled together. This results in a Van Deemter curve that is relatively flat compared to conventional single capillary columns. The benefits of a multi-capillary have been known for several decades, but have not been commercially implemented because of the difficulties in manufacturing a multi-capillary using conventional glass capillary drawing techniques.
The current state of available capillary structures also detracts from the benefits of using capillary electrophoresis (CE). Perhaps the biggest impediment to wide scale use of capillary electrophoresis is the relatively small sample sizes that are provided by the use of single capillaries. There is a great need for more flow area to provide greater analyte recovery. The total internal volume of the capillary is typically less than 1 .mu.L. This size results in a sample injection of usually less than 10 nL. For a sample containing 10 analytes, the total volume in each separated analyte is less than 1 nL, which is too small a fraction for most laboratory uses. Both liquid chromatography and gas chromatography frequently employ a mass spectrometer at the back end of a separation to determine analyte composition. The commonly minute volume of analyte recovered by CE methods leaves the mass spectrometer unable to resolve what the analyte is. The entrance of the buffer solution into the mass spectrometer and the need to have an electrical termination for the capillary column at the entrance to the ion chamber of the mass spectrometer compounds the problem of determining analyte composition with mass spectroscopy. Therefore the currently small amounts of recovered samples from CE separation push most detection equipment to or beyond their limits.
However, the performance of capillary electrophoresis (CE) begins to suffer when the diameter of most capillaries exceeds 75 microns. The 75 microns is not an absolute limitation, but the capillary diameter is dependent on the ion concentration of the buffer, applied voltage, the amount of gas contained in the ionic solution, the length of the capillary, etc. It has also been suggested that the high levels of power, i.e. voltage and current, dissipated in the capillary begins to exceed the capillary's ability to conduct away the resulting heat. Thus, as the ionic fluid becomes hotter, the desired quiescent conditions are lost and temperature runaways may result. Such runaways are believed to be responsible for non-reproducible separations.
Consequently, improved capillary structures could greatly benefit CE as well as other separation methods. For example capillary structures that could provide additional flow area for a sample without presenting problems of distribution and collection from multiple small capillaries would overcome problems of small sample recovery--one of the greater difficulties at this time with CE. Additionally, the problems of temperature runaway could be overcome by capillary structures that would provide suitable temperature control within a multiple capillary structure or a capillary structure having a relatively large capillary diameter for the ionic solution.
In particular regard to CE, it has been suggested that its performance may be enhanced by the use of multiple electrical fields at angles of 90.degree. or 120.degree. to the axial electrical field commonly associated with CE. The text "Practical Capillary Electrophoresis" mentions at page 126 the use of additional electric fields at different angles. Capillary structures that facilitate the introduction of additional electrical fields could provide an additional parameter for tailoring CE separations to the recovery of particular analytes.
U.S. Pat. No. 5,202,010 shows the use of an annular structure for improving detection of analytes recovered by CE . However, the annular structure is only used at the very end of the capillary to provide contact of an additional solution with the sample as it exits the capillary. In addition, the outer annular area shown in U.S. Pat. No. 5,202,010 is generally larger than capillary size.
It is a broad objective of this invention to provide capillary structures with improved versatility. A more specific object of this invention is to provide a single capillary tube that provides multiple capillary passages. Another specific object of this invention is to provide a single capillary tube that provides capillaries suitable for multiple functions. Another defined objective of this invention is to provide a single capillary tube containing capillary passages of different, but controlled sizes. A yet further object of this invention is to provide a single capillary tube that can provide multiple capillary passages for capillary electrophoresis. Yet another specific object of this invention is to provide a single capillary tube that has passages for heating or cooling fluids within the capillary tube or for imposing transverse electrical fields across the capillary tubes.